| 1 | .\" Automatically generated by Pod::Man v1.37, Pod::Parser v1.32 |
| 2 | .\" |
| 3 | .\" Standard preamble: |
| 4 | .\" ======================================================================== |
| 5 | .de Sh \" Subsection heading |
| 6 | .br |
| 7 | .if t .Sp |
| 8 | .ne 5 |
| 9 | .PP |
| 10 | \fB\\$1\fR |
| 11 | .PP |
| 12 | .. |
| 13 | .de Sp \" Vertical space (when we can't use .PP) |
| 14 | .if t .sp .5v |
| 15 | .if n .sp |
| 16 | .. |
| 17 | .de Vb \" Begin verbatim text |
| 18 | .ft CW |
| 19 | .nf |
| 20 | .ne \\$1 |
| 21 | .. |
| 22 | .de Ve \" End verbatim text |
| 23 | .ft R |
| 24 | .fi |
| 25 | .. |
| 26 | .\" Set up some character translations and predefined strings. \*(-- will |
| 27 | .\" give an unbreakable dash, \*(PI will give pi, \*(L" will give a left |
| 28 | .\" double quote, and \*(R" will give a right double quote. | will give a |
| 29 | .\" real vertical bar. \*(C+ will give a nicer C++. Capital omega is used to |
| 30 | .\" do unbreakable dashes and therefore won't be available. \*(C` and \*(C' |
| 31 | .\" expand to `' in nroff, nothing in troff, for use with C<>. |
| 32 | .tr \(*W-|\(bv\*(Tr |
| 33 | .ds C+ C\v'-.1v'\h'-1p'\s-2+\h'-1p'+\s0\v'.1v'\h'-1p' |
| 34 | .ie n \{\ |
| 35 | . ds -- \(*W- |
| 36 | . ds PI pi |
| 37 | . if (\n(.H=4u)&(1m=24u) .ds -- \(*W\h'-12u'\(*W\h'-12u'-\" diablo 10 pitch |
| 38 | . if (\n(.H=4u)&(1m=20u) .ds -- \(*W\h'-12u'\(*W\h'-8u'-\" diablo 12 pitch |
| 39 | . ds L" "" |
| 40 | . ds R" "" |
| 41 | . ds C` "" |
| 42 | . ds C' "" |
| 43 | 'br\} |
| 44 | .el\{\ |
| 45 | . ds -- \|\(em\| |
| 46 | . ds PI \(*p |
| 47 | . ds L" `` |
| 48 | . ds R" '' |
| 49 | 'br\} |
| 50 | .\" |
| 51 | .\" If the F register is turned on, we'll generate index entries on stderr for |
| 52 | .\" titles (.TH), headers (.SH), subsections (.Sh), items (.Ip), and index |
| 53 | .\" entries marked with X<> in POD. Of course, you'll have to process the |
| 54 | .\" output yourself in some meaningful fashion. |
| 55 | .if \nF \{\ |
| 56 | . de IX |
| 57 | . tm Index:\\$1\t\\n%\t"\\$2" |
| 58 | .. |
| 59 | . nr % 0 |
| 60 | . rr F |
| 61 | .\} |
| 62 | .\" |
| 63 | .\" For nroff, turn off justification. Always turn off hyphenation; it makes |
| 64 | .\" way too many mistakes in technical documents. |
| 65 | .hy 0 |
| 66 | .if n .na |
| 67 | .\" |
| 68 | .\" Accent mark definitions (@(#)ms.acc 1.5 88/02/08 SMI; from UCB 4.2). |
| 69 | .\" Fear. Run. Save yourself. No user-serviceable parts. |
| 70 | . \" fudge factors for nroff and troff |
| 71 | .if n \{\ |
| 72 | . ds #H 0 |
| 73 | . ds #V .8m |
| 74 | . ds #F .3m |
| 75 | . ds #[ \f1 |
| 76 | . ds #] \fP |
| 77 | .\} |
| 78 | .if t \{\ |
| 79 | . ds #H ((1u-(\\\\n(.fu%2u))*.13m) |
| 80 | . ds #V .6m |
| 81 | . ds #F 0 |
| 82 | . ds #[ \& |
| 83 | . ds #] \& |
| 84 | .\} |
| 85 | . \" simple accents for nroff and troff |
| 86 | .if n \{\ |
| 87 | . ds ' \& |
| 88 | . ds ` \& |
| 89 | . ds ^ \& |
| 90 | . ds , \& |
| 91 | . ds ~ ~ |
| 92 | . ds / |
| 93 | .\} |
| 94 | .if t \{\ |
| 95 | . ds ' \\k:\h'-(\\n(.wu*8/10-\*(#H)'\'\h"|\\n:u" |
| 96 | . ds ` \\k:\h'-(\\n(.wu*8/10-\*(#H)'\`\h'|\\n:u' |
| 97 | . ds ^ \\k:\h'-(\\n(.wu*10/11-\*(#H)'^\h'|\\n:u' |
| 98 | . ds , \\k:\h'-(\\n(.wu*8/10)',\h'|\\n:u' |
| 99 | . ds ~ \\k:\h'-(\\n(.wu-\*(#H-.1m)'~\h'|\\n:u' |
| 100 | . ds / \\k:\h'-(\\n(.wu*8/10-\*(#H)'\z\(sl\h'|\\n:u' |
| 101 | .\} |
| 102 | . \" troff and (daisy-wheel) nroff accents |
| 103 | .ds : \\k:\h'-(\\n(.wu*8/10-\*(#H+.1m+\*(#F)'\v'-\*(#V'\z.\h'.2m+\*(#F'.\h'|\\n:u'\v'\*(#V' |
| 104 | .ds 8 \h'\*(#H'\(*b\h'-\*(#H' |
| 105 | .ds o \\k:\h'-(\\n(.wu+\w'\(de'u-\*(#H)/2u'\v'-.3n'\*(#[\z\(de\v'.3n'\h'|\\n:u'\*(#] |
| 106 | .ds d- \h'\*(#H'\(pd\h'-\w'~'u'\v'-.25m'\f2\(hy\fP\v'.25m'\h'-\*(#H' |
| 107 | .ds D- D\\k:\h'-\w'D'u'\v'-.11m'\z\(hy\v'.11m'\h'|\\n:u' |
| 108 | .ds th \*(#[\v'.3m'\s+1I\s-1\v'-.3m'\h'-(\w'I'u*2/3)'\s-1o\s+1\*(#] |
| 109 | .ds Th \*(#[\s+2I\s-2\h'-\w'I'u*3/5'\v'-.3m'o\v'.3m'\*(#] |
| 110 | .ds ae a\h'-(\w'a'u*4/10)'e |
| 111 | .ds Ae A\h'-(\w'A'u*4/10)'E |
| 112 | . \" corrections for vroff |
| 113 | .if v .ds ~ \\k:\h'-(\\n(.wu*9/10-\*(#H)'\s-2\u~\d\s+2\h'|\\n:u' |
| 114 | .if v .ds ^ \\k:\h'-(\\n(.wu*10/11-\*(#H)'\v'-.4m'^\v'.4m'\h'|\\n:u' |
| 115 | . \" for low resolution devices (crt and lpr) |
| 116 | .if \n(.H>23 .if \n(.V>19 \ |
| 117 | \{\ |
| 118 | . ds : e |
| 119 | . ds 8 ss |
| 120 | . ds o a |
| 121 | . ds d- d\h'-1'\(ga |
| 122 | . ds D- D\h'-1'\(hy |
| 123 | . ds th \o'bp' |
| 124 | . ds Th \o'LP' |
| 125 | . ds ae ae |
| 126 | . ds Ae AE |
| 127 | .\} |
| 128 | .rm #[ #] #H #V #F C |
| 129 | .\" ======================================================================== |
| 130 | .\" |
| 131 | .IX Title "NEXT 3" |
| 132 | .TH NEXT 3 "2001-09-21" "perl v5.8.8" "Perl Programmers Reference Guide" |
| 133 | .SH "NAME" |
| 134 | NEXT.pm \- Provide a pseudo\-class NEXT (et al) that allows method redispatch |
| 135 | .SH "SYNOPSIS" |
| 136 | .IX Header "SYNOPSIS" |
| 137 | .Vb 1 |
| 138 | \& use NEXT; |
| 139 | .Ve |
| 140 | .PP |
| 141 | .Vb 3 |
| 142 | \& package A; |
| 143 | \& sub A::method { print "$_[0]: A method\en"; $_[0]->NEXT::method() } |
| 144 | \& sub A::DESTROY { print "$_[0]: A dtor\en"; $_[0]->NEXT::DESTROY() } |
| 145 | .Ve |
| 146 | .PP |
| 147 | .Vb 4 |
| 148 | \& package B; |
| 149 | \& use base qw( A ); |
| 150 | \& sub B::AUTOLOAD { print "$_[0]: B AUTOLOAD\en"; $_[0]->NEXT::AUTOLOAD() } |
| 151 | \& sub B::DESTROY { print "$_[0]: B dtor\en"; $_[0]->NEXT::DESTROY() } |
| 152 | .Ve |
| 153 | .PP |
| 154 | .Vb 4 |
| 155 | \& package C; |
| 156 | \& sub C::method { print "$_[0]: C method\en"; $_[0]->NEXT::method() } |
| 157 | \& sub C::AUTOLOAD { print "$_[0]: C AUTOLOAD\en"; $_[0]->NEXT::AUTOLOAD() } |
| 158 | \& sub C::DESTROY { print "$_[0]: C dtor\en"; $_[0]->NEXT::DESTROY() } |
| 159 | .Ve |
| 160 | .PP |
| 161 | .Vb 5 |
| 162 | \& package D; |
| 163 | \& use base qw( B C ); |
| 164 | \& sub D::method { print "$_[0]: D method\en"; $_[0]->NEXT::method() } |
| 165 | \& sub D::AUTOLOAD { print "$_[0]: D AUTOLOAD\en"; $_[0]->NEXT::AUTOLOAD() } |
| 166 | \& sub D::DESTROY { print "$_[0]: D dtor\en"; $_[0]->NEXT::DESTROY() } |
| 167 | .Ve |
| 168 | .PP |
| 169 | .Vb 1 |
| 170 | \& package main; |
| 171 | .Ve |
| 172 | .PP |
| 173 | .Vb 1 |
| 174 | \& my $obj = bless {}, "D"; |
| 175 | .Ve |
| 176 | .PP |
| 177 | .Vb 2 |
| 178 | \& $obj->method(); # Calls D::method, A::method, C::method |
| 179 | \& $obj->missing_method(); # Calls D::AUTOLOAD, B::AUTOLOAD, C::AUTOLOAD |
| 180 | .Ve |
| 181 | .PP |
| 182 | .Vb 1 |
| 183 | \& # Clean-up calls D::DESTROY, B::DESTROY, A::DESTROY, C::DESTROY |
| 184 | .Ve |
| 185 | .SH "DESCRIPTION" |
| 186 | .IX Header "DESCRIPTION" |
| 187 | \&\s-1NEXT\s0.pm adds a pseudoclass named \f(CW\*(C`NEXT\*(C'\fR to any program |
| 188 | that uses it. If a method \f(CW\*(C`m\*(C'\fR calls \f(CW\*(C`$self\->NEXT::m()\*(C'\fR, the call to |
| 189 | \&\f(CW\*(C`m\*(C'\fR is redispatched as if the calling method had not originally been found. |
| 190 | .PP |
| 191 | In other words, a call to \f(CW\*(C`$self\->NEXT::m()\*(C'\fR resumes the depth\-first, |
| 192 | left-to-right search of \f(CW$self\fR's class hierarchy that resulted in the |
| 193 | original call to \f(CW\*(C`m\*(C'\fR. |
| 194 | .PP |
| 195 | Note that this is not the same thing as \f(CW\*(C`$self\->SUPER::m()\*(C'\fR, which |
| 196 | begins a new dispatch that is restricted to searching the ancestors |
| 197 | of the current class. \f(CW\*(C`$self\->NEXT::m()\*(C'\fR can backtrack |
| 198 | past the current class \*(-- to look for a suitable method in other |
| 199 | ancestors of \f(CW$self\fR \*(-- whereas \f(CW\*(C`$self\->SUPER::m()\*(C'\fR cannot. |
| 200 | .PP |
| 201 | A typical use would be in the destructors of a class hierarchy, |
| 202 | as illustrated in the synopsis above. Each class in the hierarchy |
| 203 | has a \s-1DESTROY\s0 method that performs some class-specific action |
| 204 | and then redispatches the call up the hierarchy. As a result, |
| 205 | when an object of class D is destroyed, the destructors of \fIall\fR |
| 206 | its parent classes are called (in depth\-first, left-to-right order). |
| 207 | .PP |
| 208 | Another typical use of redispatch would be in \f(CW\*(C`AUTOLOAD\*(C'\fR'ed methods. |
| 209 | If such a method determined that it was not able to handle a |
| 210 | particular call, it might choose to redispatch that call, in the |
| 211 | hope that some other \f(CW\*(C`AUTOLOAD\*(C'\fR (above it, or to its left) might |
| 212 | do better. |
| 213 | .PP |
| 214 | By default, if a redispatch attempt fails to find another method |
| 215 | elsewhere in the objects class hierarchy, it quietly gives up and does |
| 216 | nothing (but see \*(L"Enforcing redispatch\*(R"). This gracious acquiesence |
| 217 | is also unlike the (generally annoying) behaviour of \f(CW\*(C`SUPER\*(C'\fR, which |
| 218 | throws an exception if it cannot redispatch. |
| 219 | .PP |
| 220 | Note that it is a fatal error for any method (including \f(CW\*(C`AUTOLOAD\*(C'\fR) |
| 221 | to attempt to redispatch any method that does not have the |
| 222 | same name. For example: |
| 223 | .PP |
| 224 | .Vb 1 |
| 225 | \& sub D::oops { print "oops!\en"; $_[0]->NEXT::other_method() } |
| 226 | .Ve |
| 227 | .Sh "Enforcing redispatch" |
| 228 | .IX Subsection "Enforcing redispatch" |
| 229 | It is possible to make \f(CW\*(C`NEXT\*(C'\fR redispatch more demandingly (i.e. like |
| 230 | \&\f(CW\*(C`SUPER\*(C'\fR does), so that the redispatch throws an exception if it cannot |
| 231 | find a \*(L"next\*(R" method to call. |
| 232 | .PP |
| 233 | To do this, simple invoke the redispatch as: |
| 234 | .PP |
| 235 | .Vb 1 |
| 236 | \& $self->NEXT::ACTUAL::method(); |
| 237 | .Ve |
| 238 | .PP |
| 239 | rather than: |
| 240 | .PP |
| 241 | .Vb 1 |
| 242 | \& $self->NEXT::method(); |
| 243 | .Ve |
| 244 | .PP |
| 245 | The \f(CW\*(C`ACTUAL\*(C'\fR tells \f(CW\*(C`NEXT\*(C'\fR that there must actually be a next method to call, |
| 246 | or it should throw an exception. |
| 247 | .PP |
| 248 | \&\f(CW\*(C`NEXT::ACTUAL\*(C'\fR is most commonly used in \f(CW\*(C`AUTOLOAD\*(C'\fR methods, as a means to |
| 249 | decline an \f(CW\*(C`AUTOLOAD\*(C'\fR request, but preserve the normal exception-on-failure |
| 250 | semantics: |
| 251 | .PP |
| 252 | .Vb 8 |
| 253 | \& sub AUTOLOAD { |
| 254 | \& if ($AUTOLOAD =~ /foo|bar/) { |
| 255 | \& # handle here |
| 256 | \& } |
| 257 | \& else { # try elsewhere |
| 258 | \& shift()->NEXT::ACTUAL::AUTOLOAD(@_); |
| 259 | \& } |
| 260 | \& } |
| 261 | .Ve |
| 262 | .PP |
| 263 | By using \f(CW\*(C`NEXT::ACTUAL\*(C'\fR, if there is no other \f(CW\*(C`AUTOLOAD\*(C'\fR to handle the |
| 264 | method call, an exception will be thrown (as usually happens in the absence of |
| 265 | a suitable \f(CW\*(C`AUTOLOAD\*(C'\fR). |
| 266 | .Sh "Avoiding repetitions" |
| 267 | .IX Subsection "Avoiding repetitions" |
| 268 | If \f(CW\*(C`NEXT\*(C'\fR redispatching is used in the methods of a \*(L"diamond\*(R" class hierarchy: |
| 269 | .PP |
| 270 | .Vb 5 |
| 271 | \& # A B |
| 272 | \& # / \e / |
| 273 | \& # C D |
| 274 | \& # \e / |
| 275 | \& # E |
| 276 | .Ve |
| 277 | .PP |
| 278 | .Vb 1 |
| 279 | \& use NEXT; |
| 280 | .Ve |
| 281 | .PP |
| 282 | .Vb 2 |
| 283 | \& package A; |
| 284 | \& sub foo { print "called A::foo\en"; shift->NEXT::foo() } |
| 285 | .Ve |
| 286 | .PP |
| 287 | .Vb 2 |
| 288 | \& package B; |
| 289 | \& sub foo { print "called B::foo\en"; shift->NEXT::foo() } |
| 290 | .Ve |
| 291 | .PP |
| 292 | .Vb 2 |
| 293 | \& package C; @ISA = qw( A ); |
| 294 | \& sub foo { print "called C::foo\en"; shift->NEXT::foo() } |
| 295 | .Ve |
| 296 | .PP |
| 297 | .Vb 2 |
| 298 | \& package D; @ISA = qw(A B); |
| 299 | \& sub foo { print "called D::foo\en"; shift->NEXT::foo() } |
| 300 | .Ve |
| 301 | .PP |
| 302 | .Vb 2 |
| 303 | \& package E; @ISA = qw(C D); |
| 304 | \& sub foo { print "called E::foo\en"; shift->NEXT::foo() } |
| 305 | .Ve |
| 306 | .PP |
| 307 | .Vb 1 |
| 308 | \& E->foo(); |
| 309 | .Ve |
| 310 | .PP |
| 311 | then derived classes may (re\-)inherit base-class methods through two or |
| 312 | more distinct paths (e.g. in the way \f(CW\*(C`E\*(C'\fR inherits \f(CW\*(C`A::foo\*(C'\fR twice \*(-- |
| 313 | through \f(CW\*(C`C\*(C'\fR and \f(CW\*(C`D\*(C'\fR). In such cases, a sequence of \f(CW\*(C`NEXT\*(C'\fR redispatches |
| 314 | will invoke the multiply inherited method as many times as it is |
| 315 | inherited. For example, the above code prints: |
| 316 | .PP |
| 317 | .Vb 6 |
| 318 | \& called E::foo |
| 319 | \& called C::foo |
| 320 | \& called A::foo |
| 321 | \& called D::foo |
| 322 | \& called A::foo |
| 323 | \& called B::foo |
| 324 | .Ve |
| 325 | .PP |
| 326 | (i.e. \f(CW\*(C`A::foo\*(C'\fR is called twice). |
| 327 | .PP |
| 328 | In some cases this \fImay\fR be the desired effect within a diamond hierarchy, |
| 329 | but in others (e.g. for destructors) it may be more appropriate to |
| 330 | call each method only once during a sequence of redispatches. |
| 331 | .PP |
| 332 | To cover such cases, you can redispatch methods via: |
| 333 | .PP |
| 334 | .Vb 1 |
| 335 | \& $self->NEXT::DISTINCT::method(); |
| 336 | .Ve |
| 337 | .PP |
| 338 | rather than: |
| 339 | .PP |
| 340 | .Vb 1 |
| 341 | \& $self->NEXT::method(); |
| 342 | .Ve |
| 343 | .PP |
| 344 | This causes the redispatcher to only visit each distinct \f(CW\*(C`method\*(C'\fR method |
| 345 | once. That is, to skip any classes in the hierarchy that it has |
| 346 | already visited during redispatch. So, for example, if the |
| 347 | previous example were rewritten: |
| 348 | .PP |
| 349 | .Vb 2 |
| 350 | \& package A; |
| 351 | \& sub foo { print "called A::foo\en"; shift->NEXT::DISTINCT::foo() } |
| 352 | .Ve |
| 353 | .PP |
| 354 | .Vb 2 |
| 355 | \& package B; |
| 356 | \& sub foo { print "called B::foo\en"; shift->NEXT::DISTINCT::foo() } |
| 357 | .Ve |
| 358 | .PP |
| 359 | .Vb 2 |
| 360 | \& package C; @ISA = qw( A ); |
| 361 | \& sub foo { print "called C::foo\en"; shift->NEXT::DISTINCT::foo() } |
| 362 | .Ve |
| 363 | .PP |
| 364 | .Vb 2 |
| 365 | \& package D; @ISA = qw(A B); |
| 366 | \& sub foo { print "called D::foo\en"; shift->NEXT::DISTINCT::foo() } |
| 367 | .Ve |
| 368 | .PP |
| 369 | .Vb 2 |
| 370 | \& package E; @ISA = qw(C D); |
| 371 | \& sub foo { print "called E::foo\en"; shift->NEXT::DISTINCT::foo() } |
| 372 | .Ve |
| 373 | .PP |
| 374 | .Vb 1 |
| 375 | \& E->foo(); |
| 376 | .Ve |
| 377 | .PP |
| 378 | then it would print: |
| 379 | .PP |
| 380 | .Vb 5 |
| 381 | \& called E::foo |
| 382 | \& called C::foo |
| 383 | \& called A::foo |
| 384 | \& called D::foo |
| 385 | \& called B::foo |
| 386 | .Ve |
| 387 | .PP |
| 388 | and omit the second call to \f(CW\*(C`A::foo\*(C'\fR (since it would not be distinct |
| 389 | from the first call to \f(CW\*(C`A::foo\*(C'\fR). |
| 390 | .PP |
| 391 | Note that you can also use: |
| 392 | .PP |
| 393 | .Vb 1 |
| 394 | \& $self->NEXT::DISTINCT::ACTUAL::method(); |
| 395 | .Ve |
| 396 | .PP |
| 397 | or: |
| 398 | .PP |
| 399 | .Vb 1 |
| 400 | \& $self->NEXT::ACTUAL::DISTINCT::method(); |
| 401 | .Ve |
| 402 | .PP |
| 403 | to get both unique invocation \fIand\fR exception\-on\-failure. |
| 404 | .PP |
| 405 | Note that, for historical compatibility, you can also use |
| 406 | \&\f(CW\*(C`NEXT::UNSEEN\*(C'\fR instead of \f(CW\*(C`NEXT::DISTINCT\*(C'\fR. |
| 407 | .Sh "Invoking all versions of a method with a single call" |
| 408 | .IX Subsection "Invoking all versions of a method with a single call" |
| 409 | Yet another pseudo-class that \s-1NEXT\s0.pm provides is \f(CW\*(C`EVERY\*(C'\fR. |
| 410 | Its behaviour is considerably simpler than that of the \f(CW\*(C`NEXT\*(C'\fR family. |
| 411 | A call to: |
| 412 | .PP |
| 413 | .Vb 1 |
| 414 | \& $obj->EVERY::foo(); |
| 415 | .Ve |
| 416 | .PP |
| 417 | calls \fIevery\fR method named \f(CW\*(C`foo\*(C'\fR that the object in \f(CW$obj\fR has inherited. |
| 418 | That is: |
| 419 | .PP |
| 420 | .Vb 1 |
| 421 | \& use NEXT; |
| 422 | .Ve |
| 423 | .PP |
| 424 | .Vb 2 |
| 425 | \& package A; @ISA = qw(B D X); |
| 426 | \& sub foo { print "A::foo " } |
| 427 | .Ve |
| 428 | .PP |
| 429 | .Vb 2 |
| 430 | \& package B; @ISA = qw(D X); |
| 431 | \& sub foo { print "B::foo " } |
| 432 | .Ve |
| 433 | .PP |
| 434 | .Vb 2 |
| 435 | \& package X; @ISA = qw(D); |
| 436 | \& sub foo { print "X::foo " } |
| 437 | .Ve |
| 438 | .PP |
| 439 | .Vb 2 |
| 440 | \& package D; |
| 441 | \& sub foo { print "D::foo " } |
| 442 | .Ve |
| 443 | .PP |
| 444 | .Vb 1 |
| 445 | \& package main; |
| 446 | .Ve |
| 447 | .PP |
| 448 | .Vb 2 |
| 449 | \& my $obj = bless {}, 'A'; |
| 450 | \& $obj->EVERY::foo(); # prints" A::foo B::foo X::foo D::foo |
| 451 | .Ve |
| 452 | .PP |
| 453 | Prefixing a method call with \f(CW\*(C`EVERY::\*(C'\fR causes every method in the |
| 454 | object's hierarchy with that name to be invoked. As the above example |
| 455 | illustrates, they are not called in Perl's usual \*(L"left\-most\-depth\-first\*(R" |
| 456 | order. Instead, they are called \*(L"breadth\-first\-dependency\-wise\*(R". |
| 457 | .PP |
| 458 | That means that the inheritance tree of the object is traversed breadth-first |
| 459 | and the resulting order of classes is used as the sequence in which methods |
| 460 | are called. However, that sequence is modified by imposing a rule that the |
| 461 | appropritae method of a derived class must be called before the same method of |
| 462 | any ancestral class. That's why, in the above example, \f(CW\*(C`X::foo\*(C'\fR is called |
| 463 | before \f(CW\*(C`D::foo\*(C'\fR, even though \f(CW\*(C`D\*(C'\fR comes before \f(CW\*(C`X\*(C'\fR in \f(CW@B::ISA\fR. |
| 464 | .PP |
| 465 | In general, there's no need to worry about the order of calls. They will be |
| 466 | left\-to\-right, breadth\-first, most\-derived\-first. This works perfectly for |
| 467 | most inherited methods (including destructors), but is inappropriate for |
| 468 | some kinds of methods (such as constructors, cloners, debuggers, and |
| 469 | initializers) where it's more appropriate that the least-derived methods be |
| 470 | called first (as more-derived methods may rely on the behaviour of their |
| 471 | \&\*(L"ancestors\*(R"). In that case, instead of using the \f(CW\*(C`EVERY\*(C'\fR pseudo\-class: |
| 472 | .PP |
| 473 | .Vb 1 |
| 474 | \& $obj->EVERY::foo(); # prints" A::foo B::foo X::foo D::foo |
| 475 | .Ve |
| 476 | .PP |
| 477 | you can use the \f(CW\*(C`EVERY::LAST\*(C'\fR pseudo\-class: |
| 478 | .PP |
| 479 | .Vb 1 |
| 480 | \& $obj->EVERY::LAST::foo(); # prints" D::foo X::foo B::foo A::foo |
| 481 | .Ve |
| 482 | .PP |
| 483 | which reverses the order of method call. |
| 484 | .PP |
| 485 | Whichever version is used, the actual methods are called in the same |
| 486 | context (list, scalar, or void) as the original call via \f(CW\*(C`EVERY\*(C'\fR, and return: |
| 487 | .IP "\(bu" 4 |
| 488 | A hash of array references in list context. Each entry of the hash has the |
| 489 | fully qualified method name as its key and a reference to an array containing |
| 490 | the method's list-context return values as its value. |
| 491 | .IP "\(bu" 4 |
| 492 | A reference to a hash of scalar values in scalar context. Each entry of the hash has the |
| 493 | fully qualified method name as its key and the method's scalar-context return values as its value. |
| 494 | .IP "\(bu" 4 |
| 495 | Nothing in void context (obviously). |
| 496 | .ie n .Sh "Using ""EVERY"" methods" |
| 497 | .el .Sh "Using \f(CWEVERY\fP methods" |
| 498 | .IX Subsection "Using EVERY methods" |
| 499 | The typical way to use an \f(CW\*(C`EVERY\*(C'\fR call is to wrap it in another base |
| 500 | method, that all classes inherit. For example, to ensure that every |
| 501 | destructor an object inherits is actually called (as opposed to just the |
| 502 | left-most-depth-first-est one): |
| 503 | .PP |
| 504 | .Vb 2 |
| 505 | \& package Base; |
| 506 | \& sub DESTROY { $_[0]->EVERY::Destroy } |
| 507 | .Ve |
| 508 | .PP |
| 509 | .Vb 3 |
| 510 | \& package Derived1; |
| 511 | \& use base 'Base'; |
| 512 | \& sub Destroy {...} |
| 513 | .Ve |
| 514 | .PP |
| 515 | .Vb 3 |
| 516 | \& package Derived2; |
| 517 | \& use base 'Base', 'Derived1'; |
| 518 | \& sub Destroy {...} |
| 519 | .Ve |
| 520 | .PP |
| 521 | et cetera. Every derived class than needs its own clean-up |
| 522 | behaviour simply adds its own \f(CW\*(C`Destroy\*(C'\fR method (\fInot\fR a \f(CW\*(C`DESTROY\*(C'\fR method), |
| 523 | which the call to \f(CW\*(C`EVERY::LAST::Destroy\*(C'\fR in the inherited destructor |
| 524 | then correctly picks up. |
| 525 | .PP |
| 526 | Likewise, to create a class hierarchy in which every initializer inherited by |
| 527 | a new object is invoked: |
| 528 | .PP |
| 529 | .Vb 6 |
| 530 | \& package Base; |
| 531 | \& sub new { |
| 532 | \& my ($class, %args) = @_; |
| 533 | \& my $obj = bless {}, $class; |
| 534 | \& $obj->EVERY::LAST::Init(\e%args); |
| 535 | \& } |
| 536 | .Ve |
| 537 | .PP |
| 538 | .Vb 6 |
| 539 | \& package Derived1; |
| 540 | \& use base 'Base'; |
| 541 | \& sub Init { |
| 542 | \& my ($argsref) = @_; |
| 543 | \& ... |
| 544 | \& } |
| 545 | .Ve |
| 546 | .PP |
| 547 | .Vb 6 |
| 548 | \& package Derived2; |
| 549 | \& use base 'Base', 'Derived1'; |
| 550 | \& sub Init { |
| 551 | \& my ($argsref) = @_; |
| 552 | \& ... |
| 553 | \& } |
| 554 | .Ve |
| 555 | .PP |
| 556 | et cetera. Every derived class than needs some additional initialization |
| 557 | behaviour simply adds its own \f(CW\*(C`Init\*(C'\fR method (\fInot\fR a \f(CW\*(C`new\*(C'\fR method), |
| 558 | which the call to \f(CW\*(C`EVERY::LAST::Init\*(C'\fR in the inherited constructor |
| 559 | then correctly picks up. |
| 560 | .SH "AUTHOR" |
| 561 | .IX Header "AUTHOR" |
| 562 | Damian Conway (damian@conway.org) |
| 563 | .SH "BUGS AND IRRITATIONS" |
| 564 | .IX Header "BUGS AND IRRITATIONS" |
| 565 | Because it's a module, not an integral part of the interpreter, \s-1NEXT\s0.pm |
| 566 | has to guess where the surrounding call was found in the method |
| 567 | look-up sequence. In the presence of diamond inheritance patterns |
| 568 | it occasionally guesses wrong. |
| 569 | .PP |
| 570 | It's also too slow (despite caching). |
| 571 | .PP |
| 572 | Comment, suggestions, and patches welcome. |
| 573 | .SH "COPYRIGHT" |
| 574 | .IX Header "COPYRIGHT" |
| 575 | .Vb 3 |
| 576 | \& Copyright (c) 2000-2001, Damian Conway. All Rights Reserved. |
| 577 | \& This module is free software. It may be used, redistributed |
| 578 | \& and/or modified under the same terms as Perl itself. |
| 579 | .Ve |